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Blood Flow in Coronary Artery Bypass Vein Grafts: Volume versus Velocity at Cardiovascular MR Imaging¹

¹ From the Departments of Cardiology (L.P.S., S.E.L., H.W.V., J.W.J., J.J.B., E.E.v.d.W.) and Radiology (L.P.S., A.d.R., H.J.L.), Leiden University Medical Center, Albinusdreef 2, 2333 ZA, Leiden, the Netherlands; the Interuniversity Cardiology Institute of the Netherlands, Utrecht (L.P.S., J.W.J., E.E.v.d.W.); and Department of Medical Statistics, Academic Medical Center, Amsterdam, the Netherlands (A.H.Z.). Received February 21, 2003; revision requested May 14; final revision received December 5; accepted January 21, 2004. J.W.J. supported by the Netherlands Heart Foundation (grant 2001D032). Address correspondence to L.P.S. (e-mail: l.p.salm@lumc.nl).

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and Methods Results Discussion REFERENCES

 
Forty-nine patients with previous bypass surgery underwent coronary angiography and cardiovascular magnetic resonance (MR) imaging of single-vein bypass grafts. Volume flow and velocity analyses were performed and compared on MR velocity maps. Bland-Altman analysis showed close agreement between the two types of analysis. Comparison of areas under the receiver operating characteristic curve revealed no significant differences between the analyses for detection of stenoses of 70% or greater. Diagnostic accuracy for volume flow and velocity parameters was 92% and 93%, respectively. Velocity analysis appears to be the preferred method, because it is less time-consuming and has a similar diagnostic accuracy to volume flow analysis.

© RSNA, 2004

Index terms: Coronary angiography, 54.1244 • Coronary vessels, bypass graft, 54.4551 • Coronary vessels, flow dynamics, 54.121412, 54.12144 • Coronary vessels, MR, 54.12144 • Coronary vessels, stenosis or obstruction, 54.76

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Materials and Methods Results Discussion REFERENCES

 
Volume flow and peak velocity measurements have been used as physiologic markers of stenosis severity in coronary arteries. Volume flow is of direct clinical value, since the quantity of blood flowing through a vessel within 1 minute is measured, and coronary flow reserve (CFR) is correlated with stenosis severity (1); however, direct invasive measurement is restricted to "open chest" procedures. Peak velocity measured by using a Doppler guidewire has been proved to correlate well with volume flow both in vitro and in vivo (2) and has evolved into a well-established method to evaluate stenoses in coronary arteries and bypass grafts in the catheterization laboratory (3,4). Cardiovascular magnetic resonance (MR) velocity mapping is a noninvasive technique used to measure coronary flow and velocity in native coronary arteries (5–8) and bypass grafts (9–12). Both volume flow and velocity can be analyzed on an MR velocity map. MR volume flow analysis has been validated as being an accurate noninvasive method of measuring volume flow (13–15). With this approach, the mean velocity for the whole luminal area of the vessel is multiplied by the luminal area. All pixels are included in the analysis; however, partial volume effects may occur at the edges of the lumen (16). Moreover, this analytical approach is time-consuming. In velocity analysis, the velocity in the center of the vessel is measured. This approach is less time-consuming and has been used successfully in several clinical studies (6,7,17,18).

The present study was performed to evaluate whether MR volume flow analysis and MR velocity analysis have comparable diagnostic accuracies in the detection of significant (70%) stenosis in single-vein coronary artery bypass grafts.

	Materials and Methods

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Patients
A total of 49 patients, who had previously undergone bypass graft surgery, underwent coronary angiography because of recurrent chest pain and, in addition, underwent MR imaging according to the study protocol. Approval of the local medical ethics committee was obtained, and all patients gave informed consent. This study was prospective and was performed in a university hospital (Leiden University Medical Center). MR-related exclusion criteria included implanted metallic devices, unstable angina, irregular heart rhythm, claustrophobia, and the inability to lie flat. Adenosine-related exclusion criteria included chronic obstructive pulmonary disease and second- or third-degree atrioventricular block. Mean age (± standard deviation) in patients was 66.4 years ± 8.7 (range, 43–79 years). There were 40 men and nine women, with mean ages of 65.7 years ± 9.1 (range, 43–79 years) and 69.7 years ± 6.3 (range, 60–77 years), respectively. Table 1 gives pertinent information regarding the 49 patients.

MR imaging.—For MR imaging, a 1.5-T imager (Gyroscan ACS-NT; Philips Medical Systems, Best, the Netherlands) equipped with a Powertrak 6000 gradient system, a cardiac research software patch, and a five-element cardiac synergy coil was used. The investigator (S.E.L.) who performed the MR imaging was blinded to the results of coronary angiography. Information regarding the coronary artery bypass procedure was used as a reference for the course of the grafts. First, gross cardiac anatomy was visualized by means of a scout image. Then, transverse electrocardiographically gated two-dimensional gradient-echo survey MR images were obtained at the level of the ascending aorta to localize the grafts. A plane perpendicular to the proximal section of the graft was planned on two differently angled survey images, and, for MR velocity mapping, a fast breath-hold turbo-field echo-planar imaging sequence was used at rest and during stress (140 µg of adenosine per kilogram of body weight per minute administered intravenously). The turbo-field echo-planar imaging sequence included the following parameters: 11.0/4.6 (repetition time msec/echo time msec), flip angle of 20°, temporal resolution of 23 msec, field of view of 200 x 100 mm, data acquisition matrix of 128 x 60, in-plane spatial resolution of 1.6 x 1.6 mm reconstructed to 0.8 x 0.8 mm by means of zero filling of k space, section thickness of 6 mm, imaging duration of 20 heartbeats, velocity encoding of 75 cm/sec, and prospective electrocardiographic triggering (9,10).

Volume flow analysis and velocity analysis were performed with an analytic software package (FLOW; Medis, Leiden, the Netherlands) by the same investigator (S.E.L.), and the flow and velocity analyses of the same velocity map were separated by a minimum of 4 weeks to avoid bias. The duration (in minutes) of the volume flow and velocity analyses per patient was registered from the start of image analysis to report generation.

For the volume flow analysis, the luminal area was traced manually on the modulus image and transferred to the phase image (Fig 1). The position and size of each contour were adjusted according to the cardiac phase. The flow rate (in milliliters per second) was calculated by multiplying the average velocity over the luminal area with the luminal area for each cardiac phase. Flow rate–versus-time curves were reconstructed, and the volume flow rate (in milliliters per minute) was obtained by multiplying the integrated volumetric flow rate per heartbeat with the heart rate (Fig 2). Systolic peak flow rate and diastolic peak flow rate were defined as maximal flow rate during systole and diastole, respectively, both measured in milliliters per second. CFR was calculated as the ratio of volume flow during adenosine-induced stress and volume flow at rest. The ratio between diastolic and systolic peak flow rates was regarded as the diastolic-to-systolic flow ratio.

View larger version (150K): [in this window] [in a new window] [Download PPT slide]   Figure 1. Sagittal oblique images from a typical phase-contrast MR examination. Modulus (top) and phase (bottom) images, obtained with a breath-hold turbo-field echo-planar (11.0/4.6) sequence, show the cross section of a vein graft (arrow) to the left anterior descending coronary artery and an arterial bypass graft (arrowhead) to the first diagonal branch during late diastole. Enlargements of the vein graft cross section (right) are shown to illustrate the volume flow and velocity analyses. PA = pulmonary artery.

View larger version (40K): [in this window] [in a new window] [Download PPT slide]   Figure 2. Top: Flow rate-versus-time curve. The integral of the curve (hatched area) indicates the volume flow per heartbeat. The volume flow per heartbeat multiplied with the heart rate results in the volume flow per minute. DPF = diastolic peak flow, SPF = systolic peak flow. Bottom: Velocity-versus-time curve from the same bypass graft displays the velocity parameters that were distracted from the curve. APV = average peak velocity, DPV = diastolic peak velocity, SPV = systolic peak velocity. Flow rate- and velocity-versus-time curves were used to quantify volume flow and velocity parameters.

Statistical Analysis
Paired correlations of the flow and velocity parameters were determined by calculating the Pearson correlation coefficient. Limits of agreement between the volume flow and velocity analyses were evaluated by means of Bland-Altman analysis (21). Receiver operating characteristic (ROC) curve analysis was employed to determine the diagnostic performance of each flow and velocity parameter for demonstrating a significant stenosis by using the Simpson rule (univariate analysis).

Significant parameters from the univariate analysis were subsequently used to perform stepwise multivariate logistic regression and ROC curve analyses to determine the diagnostic performance of the combined set of flow or velocity parameters, respectively, in the detection of a significant stenosis (multivariate analysis). Multiple single-vein grafts in the same patient were possibly correlated (22), for instance, because they share the same risk of stenosis due to systemic factors. This correlation was investigated for the calculation of the correlations between the flow and velocity parameters and for the logistic regression analysis by using a robust estimator of the standard errors and by the subsequent calculation of the P values. Results indicated no correlation between multiple grafts within a patient. Therefore, all grafts were treated independently for calculating correlations and areas under the curve in ROC analysis and for comparing ROC curves. The optimal cutoff values for flow and velocity parameters needed to predict stenosis were defined as those providing the maximal sum of sensitivity and specificity. Sensitivity, specificity, and diagnostic accuracy were based on their standard definitions and are presented with 95% confidence intervals.

For all tests, P < .05 was considered to indicate a statistically significant difference.

	Results

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A total of 80 single-vein grafts were analyzed. Sixteen grafts supplied the left anterior descending artery territory; 25 grafts, the left circumflex artery territory; and 39 grafts, the right coronary artery territory. By means of quantitative coronary arteriography, severity of stenosis was determined to range from 0% to 100% with a mean of 50% ± 39. Stenosis of 70% or greater was detected in 25 grafts (31%).

At MR imaging, 22 of 80 grafts were not visible in their expected course, and these grafts were regarded as occluded. MR velocity mapping was performed in the remaining 58 grafts. Baseline velocity mapping was unsuccessful in one graft because of artifacts. Adenosine-induced stress velocity mapping was not possible in eight grafts because of side effects from the adenosine.

Correlations for the volume flow and velocity parameters are shown in Table 2. Highly significant correlations were found for all paired parameters both at rest and during stress (P < .01 for all correlations).

View larger version (22K): [in this window] [in a new window] [Download PPT slide]   Figure 3. Bland-Altman plot of CFR and CFVR derived from the volume flow and velocity analysis, respectively. Solid horizontal line shows the mean difference between CFR and CFVR, and dashed lines are ± 2 standard deviations. The plot shows a close agreement between the two analysis methods. The mean difference between CFVR and CFR was not significant.

View larger version (20K): [in this window] [in a new window] [Download PPT slide]   Figure 4. Graph shows ROC curves of single-vein grafts with luminal stenosis 70% or greater derived from flow and velocity parameters. When the area under the ROC curve for flow parameters was compared with that for velocity parameters, no significant differences were found. Optimal cutoff points are indicated by arrows.

	Discussion

TOP ABSTRACT INTRODUCTION Materials and Methods Results Discussion REFERENCES

 
In the present study, two approaches used to analyze MR velocity maps, MR volume flow analysis and MR velocity analysis, were compared in a head-to-head fashion in single-vein bypass grafts. Close agreement was shown between the volume flow and velocity methods when comparing CFR, CFVR, diastolic-to-systolic flow ratio, and diastolic-to-systolic velocity ratio. Fair sensitivity, specificity, and diagnostic accuracy were shown for both approaches in depicting significant (70%) stenosis. Therefore, a single approach would be sufficient when analyzing MR velocity maps.

The concept of CFR was proposed by Gould et al (1) in the early 1970s, when the necessity arose for a hemodynamic assessment of coronary stenoses visualized on the coronary angiogram. Absolute coronary blood flow could be measured only by using perivascular flow transducers in "open chest" procedures. CFR was therefore validated in animal models or in patients undergoing bypass graft surgery (1,23,24); however, extensive use in the clinical setting was not possible. When the diameter of intravascular catheter-based Doppler ultrasonographic devices could be reduced to an 0.018-inch diameter, it became feasible to measure absolute velocity of blood flow and calculate CFVR in coronary arteries in patients during catheterization. Absolute blood flow correlated well with Doppler-derived absolute velocity and volume flow both in vitro and in vivo (2,25,26). Doppler-derived velocity and CFVR proved their potential in numerous clinical applications, such as in the identification of hemodynamically significant stenoses in native coronary arteries and vein grafts (3,4), in the functional assessment of stenoses of intermediate severity (27), in the determination of the need for and the outcome after coronary intervention (28–30), and in the prediction of restenosis (31).

With MR imaging and a velocity mapping sequence, both CFR and CFVR can be calculated, as was described in Materials and Methods. In clinical studies, both the volume flow analysis (8,5,12) and the velocity analysis (7,17,18) have been used successfully. In our study, the accuracies of volume flow and velocity analyses were not significantly different when evaluating MR velocity maps. Other factors, such as errors in phase-contrast MR imaging and the time necessary to analyze the MR velocity maps, become important in choosing a method for analysis.

When obtaining flow measurements at phase-contrast MR imaging, several errors might occur (32). Some errors are dependent on the prescription of the MR velocity mapping sequence (eg, inadequate spatial and temporal resolution, mismatched encoding velocity, deviation of the perpendicular imaging plane from the flow direction). In the present study, an effort was made to keep the effects of errors minimal. Encoding velocity was adequately matched to avoid aliasing, and two survey images obtained at different angles were used to plan the imaging plane perpendicular to the graft. Phase-offset errors are dependent on local magnetic-field inhomogeneities and are usually small. In the analysis of MR velocity maps, all of these errors affect both volume flow and velocity analyses.

In volume flow analysis, all pixels in the cross-sectional area of the graft lumen are included. Pixels on the vessel edge may average signals both from the vessel lumen and from surrounding tissue. This type of error, known as partial volume effect, could either increase or decrease apparent flow (13,33). The extent of partial volume effect depends predominantly on spatial resolution and relative signal intensity of stationary tissue (16). With the current MR imaging techniques, spatial resolution for flow measurements in small vessels is limited and, due to background noise velocity, is not zero in stationary tissue, enhancing partial volume effects. However, velocities at the vessel edge are expected to be low, as there is a pulsatile laminar flow pattern in bypass grafts with the highest velocity in the center of the vessel (34). Since diagnostic accuracy of the volume flow and velocity analyses did not differ significantly in our study, this might indicate that this type of error has a relatively minor role when MR imaging is used to measure flow in single-vein bypass grafts.

In our study, a volume flow analysis required 25.9 minutes ± 4.3 to complete, whereas a velocity analysis required only 11.1 minutes ± 2.2. If clinical acceptance of a diagnostic test is to be achieved, performance and analysis of the test should be fast and straightforward. Since the diagnostic accuracy from both analyses was similar, velocity analysis appears to be the preferable method.

Our study had limitations in that other issues associated with MR velocity mapping, such as reference values of the MR flow and velocity parameters, spatial and temporal resolution of the acquired MR images, and proximal location of the imaging plane in the vessel were not addressed; this is because our purpose was to compare volume flow and velocity analyses of MR velocity maps. In addition, the sample size of our study was small. The study was not designed to test equivalence of the analysis methods. To demonstrate equivalence, a larger data set would be necessary.

In the evaluation of MR velocity maps of single-vein coronary artery bypass grafts, there is close agreement between volume flow and velocity analyses. For detection of a stenosis 70% or greater, velocity analysis has a diagnostic accuracy similar to that of volume flow analysis, and it is less time-consuming. Therefore, velocity analysis appears to be the method of preference in the analysis of MR velocity maps of bypass grafts.

	FOOTNOTES

 
Authors stated no financial relationship to disclose.

Abbreviations: CFR = coronary flow reserve, CFVR = coronary flow velocity reserve, ROC = receiver operating characteristic

Author contributions: Guarantor of integrity of entire study, H.J.L.; study concepts, S.E.L., H.W.V., J.W.J., E.E.v.d.W., A.d.R., H.J.L.; study design, L.P.S., S.E.L., H.W.V., J.W.J.; literature research, L.P.S.; clinical studies, L.P.S., S.E.L., H.W.V., J.W.J.; data acquisition, L.P.S., S.E.L., H.J.L.; data analysis/interpretation, L.P.S., S.E.L., J.J.B., A.H.Z., H.J.L.; statistical analysis, L.P.S., A.H.Z.; manuscript preparation, L.P.S., S.E.L., J.J.B., A.H.Z., H.J.L.; manuscript definition of intellectual content, H.W.V., J.W.J., E.E.v.d.W., A.d.R.; manuscript editing, L.P.S.; manuscript revision/review, L.P.S., H.W.V., J.J.B., E.E.v.d.W., A.d.R., H.J.L.; manuscript final version approval, all authors

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TOP ABSTRACT INTRODUCTION Materials and Methods Results Discussion REFERENCES

 

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This Article Abstract Figures Only Full Text (PDF) All Versions of this Article: 2323030289v1 232/3/915    most recent Submit a response Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Salm, L. P. Articles by Lamb, H. J. Search for Related Content PubMed PubMed Citation Articles by Salm, L. P. Articles by Lamb, H. J.